text: In the hydrodynamic entrance region, the wall shear stress, , is highest at the pipe inlet, where the boundary layer thickness is the smallest. Shear stress decreases along the flow direction. That is why the pressure drop is highest in the entrance region of a pipe, which increases the average friction factor for the whole pipe. This increase in the friction factor is negligible for long pipes. In a fully developed region, the pressure gradient and the shear stress in flow are in balance.